The present invention is directed to pulps useful for making lyocell-molded bodies, including films, fibers, and non-woven webs, and to methods of making such pulps useful for making the lyocell-molded bodies, to the lyocell-molded bodies made from the pulps and to the methods for making the lyocell-molded bodies. In particular, the present invention is directed to using xe2x80x9cyoungxe2x80x9d wood (often characterized as xe2x80x9ccore woodxe2x80x9d, xe2x80x9cjuvenile woodxe2x80x9d, xe2x80x9clow specific gravity woodxe2x80x9d or, in some cases as xe2x80x9cthinningsxe2x80x9d.).
Cellulose is a polymer of D-glucose and is a structural component of plant cell walls. These cells are referred to as fibers. Cellulosic fibers are especially abundant in tree trunks from which they are extracted, converted into pulp, and thereafter utilized to manufacture a variety of products.
Rayon is the name given to a fibrous form of regenerated cellulose that is extensively used in the textile industry to manufacture articles of clothing. For over a century, strong fibers of rayon have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. The cellulose is then reacted with carbon disulfide to form cellulose xanthate, which is then dissolved in dilute caustic soda solution. After filtration and deaeration, the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, and zinc sulfate to form continuous filaments. The resulting viscose rayon is presently used in textiles and was formerly widely used for reinforcing rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammonia copper oxide. This property forms the basis for production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers, which are then decoppered and washed. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
The foregoing processes for preparing rayon both require that the cellulose be chemically derivatized or complexed in order to render it soluble and therefore capable of being spun into fibers. In the viscose process, the cellulose is derivatized, while in the cuprammonium rayon process, the cellulose is complexed. In either process, the derivatized or complexed cellulose must be regenerated and the reagents used to solubilize it must be removed. The derivatization and regeneration steps in the production of rayon significantly add to the cost of this form of cellulose fiber. Consequently, in recent years attempts have been made to identify solvents that are capable of dissolving underivatized cellulose to form a dope of underivatized cellulose from which fibers can be spun.
One class of organic solvents useful for dissolving cellulose are the amine N-oxides, in particular the tertiary amine N-oxides. For example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a cellulose fiber precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. Several manufacturers presently produce lyocell fibers, principally for use in the textile industry. For example, Acordis, Ltd. presently manufactures and sells a lyocell fiber called Tencel(copyright) fiber.
Currently available lyocell fibers are produced from wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose. These highly processed pulps are referred to as dissolving grade or high alpha (or high xcex1) pulps, where the term alpha (or xcex1) refers to the percentage of cellulose. Thus, a high alpha pulp contains a high percentage of cellulose, and a correspondingly low percentage of other components, especially hemicellulose. The processing required to generate a high alpha pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom.
Since the conventional Kraft process stabilizes residual hemicelluloses against further alkaline attack, it is not possible to obtain acceptable high alpha pulps for lyocell products, through subsequent treatment of Kraft pulp in the conventional bleaching stages. In order to prepare high alpha pulps by the Kraft process, it is necessary to pretreat the wood chips in an acid phase before the alkaline pulping stage. A significant amount of material, primarily hemicellulose, on the order of 10% or greater of the original wood substance, is solubilized in this acid phase pretreatment and thus process yields drop. Under these conditions, the cellulose is largely resistant to attack, but the residual hemicelluloses are degraded to a much shorter chain length and are therefore removed to a large extent in the subsequent Kraft cook by a variety of hemicellulose hydrolysis reactions or by dissolution. The disadvantage of conventional high alpha pulps used for lyocell is the resulting loss of yield by having to eliminate hemicelluloses.
In view of the expense of producing commercial high alpha pulps, it would be desirable to have alternatives to conventional high alpha pulps for making lyocell products. In addition, manufacturers would like to minimize the capital investment necessary to produce such types of pulps by utilizing existing capital plants. Thus, there is a need for relatively inexpensive, low alpha (e.g., high yield, high hemicellulose) pulps that have attributes that render them useful in lyocell-molded body production.
In U.S. Pat. No. 6,210,801, fully incorporated herein by reference in its entirety, assigned to the assignee of the present application, low viscosity, high hemicellulose pulp is disclosed that is useful for lyocell-molded body production. The pulp is made by reducing the viscosity of the cellulose without substantially reducing the hernicellulose content. Such processes use an acid, or an acid substitute, or other methods therein described.
While the methods described in the xe2x80x2801 patent are effective at reducing the average degree of polymerization (D.P.) of cellulose without substantially decreasing the hemicellulose content, a further need existed for a process that did not require a separate copper number reducing step and which was readily adaptable to pulp mills that have oxygen reactors, multiple alkaline stages and/or alkaline conditions suitable for substantial D.P. reduction of bleached or semi-bleached pulp. Environmental concerns have also generated a great interest in using bleaching agents that reduce the use of chlorine compounds. In recent years, the use of oxygen as a delignifying agent has occurred on a commercial scale. Examples of equipment and apparatus useful for carrying out an oxygen stage delignification process are described in U.S. Pat. Nos. 4,295,927; 4,295,925; 4,298,426; and 4,295,926. In U.S. Pat. No. 6,331,554, assigned to the assignee of the present application, fully incorporated herein by reference in their entirety, a high hemicellulose, low viscosity pulp is disclosed that is useful for lyocell-molded body formation. The pulp is made from an alkaline pulp by treating the alkaline pulp with an oxidizing agent in a medium to high consistency reactor to reduce the D.P. of the cellulose, without substantially reducing the hemicellulose or increasing the copper number.
Further efforts to reduce the cost of making lyocell-molded bodies has resulted in U.S. application Ser. No. 09/842,274, U.S. Pat. No. 6,605,350, fully incorporated by reference in its entirety. In the xe2x80x2274 application, the assignee of the present invention describes pulps made from sawdust and other low fiber length wood using a procedure similar to that of the xe2x80x2554 patent. These pulps are high in hemicellulose and low in viscosity, which makes them especially suitable for lyoccll-molded body formation.
The forest industry continues to generate vast quantities of byproducts in the normal course of day-to-day forestry management and wood processing. These byproducts are for the most part underutilized. The need to conserve resources by utilizing wood byproducts in new ways presents a unique opportunity. It would be advantageous to develop a low cost pulp that is useful for making lyocell-molded bodies from all this underutilized wood, namely from the core wood or young or juvenile wood such as thinnings, hereafter referred to as low specific gravity wood. Thus, presenting a low cost alternative to the highly refined high-alpha pulps.
One embodiment of the invention is a pulp having at least 7% by weight hemicellulose; a viscosity of less than or about 32 cP; a copper number less than or about 2; a weighted average fiber length less than or about 2.7 mm; and a coarseness less than or about 23 mg/100 m. In another embodiment of the invention, a method for making lyocell-molded body is provided. The method includes dissolving a pulp in a solvent to form a cellulose solution; forming a lyocell-molded body from the solution; and regenerating the molded body, wherein the pulp has at least 7% by weight hemicellulose, a viscosity less than or about 32 cP; a copper number less than or about 2; a weighted average fiber length less than or about 2.7 mm; and a coarseness less than or about 23 mg/100 m. The method can use a meltblowing, centrifugal spinning, spun bonding, or dry-jet wet technique.
In another embodiment of the invention, a method of making a pulp is provided. The method includes pulping of wet material with a specific gravity less than or about 0.41 using an alkaline pulping process; and bleaching the pulp to reduce the viscosity of the pulp to or about 32 cP or lower. The bleached pulp has at least 7% hemicellulose by weight, a copper number less than or about 2, a weighted average fiber length less than or about 2.7 mm, and a coarseness less than or about 23 mg/100 m.
In another embodiment of the invention, a lyocell product is provided. The lyocell product has at least 7% hemicellulose by weight, and cellulose, wherein the pulp used to make the product has a viscosity less than or about 32 cP, a copper number less than or about 2, a weighted average fiber length less than or about 2.7 mm, and a coarseness less than or about 23 mg/100 m. Lyocell products can be fibers, films, or non-woven webs, for example.
The use of low specific gravity wood can produce a lower brownstock viscosity for a given kappa number target. Using wood with low specific gravity values reduce the bleach stage temperature and the chemical dose needed in the bleach plant to produce pulp having acceptable lyocell specifications. Low specific gravity wood results in very low viscosity levels without increasing the copper number of the pulp or the concentration of carbonyl in the pulp above acceptable levels. The process does not use an acid phase pretreatment prior to pulping, and the subsequent bleaching conditions do not result in a substantial decrease in hemicellulose content.